Gas lance system for molten metal furnace

ABSTRACT

A lance system for lancing gas into a molten metal furnace wherein primary gas is passed into a furnace as a coherent jet enveloped in secondary gas provided into the furnace through a plurality of secondary openings communicating with respective secondary passages having restrictions within the lance set back from the lance face.

TECHNICAL FIELD

[0001] This invention relates generally to coherent jet technology and,more particularly, to the application of coherent jet technology in amolten metal furnace.

BACKGROUND ART

[0002] A recent significant advancement in the field of gas dynamics isthe development of coherent jet technology which produces a laser-likejet of gas which can travel a long distance while still retainingsubstantially all of its initial velocity and with very little increaseto its jet diameter. One very important commercial use of coherent jettechnology is for the introduction of gas into liquid, such as moltenmetal, whereby the gas lance may be spaced a large distance from thesurface of the liquid, enabling safer operation as well as moreefficient operation because much more of the gas penetrates into theliquid than is possible with conventional practice where much of the gasdeflects off the surface of the liquid and does not enter the liquid.

[0003] In a coherent gas jet system, one or more gas jets are surroundedby a flame envelope to maintain coherency over a long distance from theinjection lance. When a coherent gas jet system is employed in a harshenvironment such as a molten metal furnace, material within the harshenvironment may plug some of the apertures on the lance from which gasis provided for forming the flame envelope. This requires periodicstoppage of operations and cleaning of the lance. This stoppage reducesthe efficiency of the industrial operation, e.g. electric arc furnacepractice or basic oxygen furnace practice, in which the coherent gas jetsystem is employed.

[0004] Accordingly it is an object of this invention to provide a systemfor establishing a coherent gas jet wherein plugging or fouling of theapertures or ports for the provision of the flame envelope gases isreduced or eliminated.

SUMMARY OF THE INVENTION

[0005] The above and other objects, which will become apparent to thoseskilled in the art upon a reading of this disclosure, are attained bythe present invention, one aspect of which is:

[0006] A lance comprising a lance face having at least one primaryopening for passing at least one gas jet out from the lance, and aplurality of secondary openings arranged around the primary opening(s)on the lance face, each secondary opening communicating with a passagefor passing secondary gas within the lance to the respective secondaryopening, and each said passage having a restriction therein so that thearea for gas flow at the restriction is smaller than the area of thesecondary opening with which that passage communicates.

[0007] Another aspect of the invention is:

[0008] A method for providing at least one gas jet from a lance into amolten metal furnace comprising:

[0009] (A) passing primary gas in at least one primary gas jet out froma lance to form at least one primary gas jet within the molten metalfurnace;

[0010] (B) passing secondary gas through a plurality of passages withinthe lance, each passage communicating with a secondary opening andhaving a restriction therein, and said secondary gas flowing within apassage having a higher pressure at the entrance to the restriction thanat the secondary opening; and

[0011] (C) passing secondary gas out from the secondary openings andforming a gas envelope around the primary gas jet(s) within the moltenmetal furnace.

[0012] As used herein the term “coherent jet” means a gas jet which isformed by ejecting gas from a nozzle and which has a velocity andmomentum profile along its length which is similar to its velocity andmomentum profile upon ejection from the nozzle. Another way ofdescribing a coherent jet is a gas jet which has little or no change indiameter along its length.

[0013] As used herein the term “length” when referring to a coherent gasjet means the distance from the nozzle from which the gas is ejected tothe intended impact point of the coherent gas jet or to where the gasjet ceases to be coherent.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a head on view of one preferred embodiment of a lanceand

[0015]FIG. 2 is a cross sectional view of that embodiment of the lancewhich may be used in the practice of this invention.

[0016]FIG. 3 illustrates the embodiment of the invention illustrated inFIGS. 1 and 2 in operation.

[0017]FIG. 4 is a detailed view of one preferred embodiment of therestriction in the secondary gas passage in the practice of thisinvention.

[0018] The numerals in the Drawings are the same for the commonelements.

DETAILED DESCRIPTION

[0019] The invention will be described in detail with reference to theDrawings.

[0020] Referring now to FIGS. 1, 2, 3 and 4, primary gas 2 is passedwithin lance 1 through primary gas passage or channel 26 and thenthrough a nozzle 20, preferably a converging/diverging nozzle, and thenout from lance 1 through primary gas opening 11 to form primary gas jetor coherent gas jet stream 30. Typically the velocity of the primary gasjet is within the range of from 500 to 3000 feet per second (fps).Preferably the velocity of the primary gas jet is supersonic when it isformed upon ejection from the lance face and remains supersonic for adistance of at least 20 d where d is the exit diameter of nozzle 20.Although the Drawings illustrate an embodiment employing only oneprimary gas jet or coherent gas jet injected into a molten metal furnacefrom the lance, more than one primary gas jet may be injected from thelance in the practice of this invention. When more than one primary orcoherent gas jet from the lance are employed, generally the number ofsuch primary or coherent gas jets is within the range of from 2 to 6.

[0021] Any effective gas may be used as the primary gas in the practiceof this invention. Among such gases one can name oxygen, nitrogen,argon, carbon dioxide, hydrogen, helium, steam and hydrocarbon gases.Also mixtures comprising two or more gases, e.g. air, may be used as thegas in the practice of this invention.

[0022] Secondary gas is passed through the lance and out from the lancethrough a plurality of secondary openings arranged on the lance face 5around the primary opening(s) so that the secondary gas forms a gasenvelope or gas shroud around and along the length of the primary gasjet(s) provided from the lance. The gas envelope combusts to form aflame envelope around and along the length of the primary gas jet(s).Preferably, as illustrated in the Drawings, two secondary gases, fueland oxidant, are provided in this manner from the lance, and these twosecondary gases mix and combust to form the flame envelope around andalong the length of the primary gas jet(s).

[0023] Fuel and oxidant are provided from lance 1 from one or more setsof secondary openings. The fuel is preferable gaseous and may be anyfuel such as methane or natural gas. The oxidant may be air,oxygen-enriched air having an oxygen concentration exceeding that ofair, or commercial oxygen having an oxygen concentration of at least 90mole percent. Preferably the oxidant is a fluid having an oxygenconcentration of at least 25 mole percent. The fuel and oxidant may bepassed out from one set of secondary openings as a mixture or may bepassed individually as fuel and oxidant from respective secondaryopenings on the lance face. The embodiment of the invention illustratedin the Drawings is a preferred embodiment wherein, when the primary gasis oxygen, the fuel and oxidant are passed out from lance 1 through aninner ring or circle of secondary openings 9 for the provision of fueland through an outer ring or circle of secondary openings 10 for theprovision of oxidant. The diameter (ds) of each of the secondaryopenings, and thus the area for gas flow for each secondary opening, issmaller than the diameter (d) of each of the primary opening(s).

[0024] Fuel 3 is passed within lance 1 through inner annular passage 25which is annular to primary gas passage 26, and oxidant 4 is passedwithin lance 1 through outer annular passage 27, which is annular toprimary gas passage 26 and inner annular passage 25. Proximate the face5 of lance 1, typically at a distance of at least 3 ds from the face 5of lance 1, inner annular passage 25 communicates with a plurality ofsecondary passages 7 which each communicate with a secondary opening 9on lance face 5, and outer annular passage 27 communicates with aplurality of secondary passages 8 which each communicate with asecondary opening 10 on lance face 5. In this way fuel passes throughlance 1 through inner annular passage 25 and then through secondarypassages 7 and out from the lance through secondary openings 9 whileoxidant passes through lance 1 through outer annular passage 27 and thenthrough secondary passages 8 and out from the lance through secondaryopenings 10. The fuel and oxidant passed out from the lance form a gasenvelope around the primary gas jet(s) which combusts to form a flameenvelope or flame shroud 33 around the primary gas jet(s) within themolten metal furnace. Flame envelope 33 around primary gas stream 30serves to keep ambient gas from being drawn into the gas stream 30,thereby keeping the velocity of gas stream 30 from significantlydecreasing and keeping the diameter of the gas stream 30 fromsignificantly increasing, for at least a distance of 20 d from thenozzle exit. That is, the flame envelope or flame shroud serves toestablish and maintain gas stream 30 as a coherent jet for a distance ofat least 20 d from the nozzle exit.

[0025] Due to the relatively small size of the secondary openings, whichtypically have a diameter within the range of from 0.15 to 0.75 inch,the secondary opening may be partially or even totally blocked bymaterial, such as a molten metal or slag, from within the molten metalfurnace. This may occur by globs of molten material being pasted oversome holes. It may also occur due to pressure surges in the furnacewhich can create a momentary reverse flow in the auxiliary gas holes.This carries small molten particles into the holes, and these deposit onthe walls of the holes, gradually closing them off. Such partial ortotal blockage vitiates the integrity of the flame envelope thusreducing its efficacy which has a deleterious effect on the coherentprimary gas jet(s) passed out from the lance.

[0026] The pressure to push away a plug at the exit of an auxiliary gashole, or to prevent a reverse flow due to a pressure surge in thefurnace, can only go as high as the pressure in the annular feedchannels 25 and 27. This may be only a few pounds per square inch. Inorder to provide a higher pressure for this purpose each secondarypassage has a restriction which reduces the gas flow area at therestriction so that it is less than the gas flow area of the secondaryopening with which that secondary passage communicates. The effect ofthis is to require a higher pressure in the annular gas channels and atthe entrance to the restrictions in order to maintain the design flow.The available pressure in the auxiliary gas holes is correspondinglyincreased to provide more pressure to push away plugs and to resistpressure surges in the furnace.

[0027] Generally the gas flow area at the restriction within thesecondary passage is from 20 to 80 percent of the gas flow area of thesecondary opening with which that secondary passage communicates. Thesecondary gas passing through the restrictions generally requires 50 to1000 percent higher inlet pressure (compared to the pressure required toachieve this flow without the restrictions) to achieve the designed flowthrough the restrictions and subsequent secondary openings. Even higherpressures can be advantageously used, but may not generally beavailable. This serves to reduce the occurrence of secondary openingplugging since, in the event that one or more of the secondary openingsbecomes blocked by metal or slag splash or other foreign matter, the 50to 1000 percent higher pressure in the annular passages 25 and 27 isbetter able to dislodge and clear the blockage or prevent it fromoccurring in the first place.

[0028] The downstream termination of the restriction within eachsecondary passage is preferably located at a distance of at least 3 dsupstream from the exit openings of secondary openings 9 and 10 to allowthe secondary gas to expand and reestablish fully developed flowconditions with pressure and velocity that are characteristic of thesecondary passage without a restriction. One convenient location forrestrictions 35 and 36 is shown in FIGS. 2 and 3 as at the entrance ofthe secondary passages. If this location will result in a hole lengthsubstantially greater than 5 ds, it is desirable to locate therestrictions within the hole about 5 ds from the exit, as shown in FIG.4. The smaller volume downstream of the restriction will result in afaster pressure buildup to rapidly resist plugging and reverse flow.

[0029] Although the invention has been described in detail withreference to a certain preferred embodiment, those skilled in the artwill recognize that there are other embodiments of the invention withinthe spirit and the scope of the claims.

1. A lance comprising a lance face having at least one primary opening for passing at least one gas jet out from the lance, and a plurality of secondary openings arranged around the primary opening(s) on the lance face, each secondary opening communicating with a passage for passing secondary gas within the lance to the respective secondary opening, and each said passage having a restriction therein so that the area for gas flow at the restriction is smaller than the area of the secondary opening with which that passage communicates.
 2. The lance of claim 1 wherein the area for gas flow at the restriction is within the range of from 20 to 80 percent of the area of the secondary opening.
 3. The lance of claim 1 wherein the plurality of secondary openings are arranged in at least one ring around the primary opening(s).
 4. The lance of claim 1 wherein the plurality of secondary openings are arranged in two rings, an inner ring and an outer ring, around the primary opening(s).
 5. The lance of claim 4 wherein the passages communicating with the secondary openings arranged on the inner ring communicate with an inner annular passage within the lance, and the passages communicating with the secondary openings arranged on the outer ring communicate with an outer annular passage within the lance.
 6. The lance of claim 1 wherein the downstream termination of each restriction is a distance of at least 3 ds from the lance face where ds is the diameter of the secondary opening corresponding to that restriction.
 7. A method for providing at least one gas jet from a lance into a molten metal furnace comprising: (A) passing primary gas in at least one primary gas jet out from a lance to form at least one primary gas jet within the molten metal furnace; (B) passing secondary gas through a plurality of passages within the lance, each passage communicating with a secondary opening and having a restriction therein, and said secondary gas flowing within a passage having a higher pressure at the entrance to the restriction than at the secondary opening; and (C) passing secondary gas out from the secondary openings and forming a gas envelope around the primary gas jet(s) within the molten metal furnace.
 8. The method of claim 7 wherein the gas envelope comprises a flame envelope.
 9. The method of claim 7 employing one primary gas jet.
 10. The method of claim 7 employing from 2 to 6 primary gas jets.
 11. The method of claim 7 wherein the primary gas jet(s) comprises oxygen.
 12. The method of claim 7 wherein the primary gas jet(s) has a supersonic velocity.
 13. The method of claim 7 wherein the secondary gas pressure at the entrance to the restriction exceeds the secondary gas pressure at the secondary opening by at least 50 percent. 